28mm Copper Pipe: When You Need It, How It Joins, and What It Costs

Your plumber says the boiler needs 28mm copper for the flow and return. You nod, because what else are you going to do? But 28mm pipe costs roughly 2.5 times more per metre than 15mm. If your plumber runs 28mm where 22mm would have been perfectly adequate, that's money wasted on pipe, fittings, and labour for no benefit. Knowing when 28mm is genuinely required, and when it isn't, is the difference between a well-specified system and an over-engineered one.

What it is and what it's for

28mm copper pipe is tube with an external diameter of 28mm, an internal bore of 26.2mm, and a wall thickness of 0.9mm. It conforms to BS EN 1057 (the British/European standard for copper tube used in domestic heating and water supply), supplied in R250 grade (half-hard temper, meaning it's rigid enough to run in straight lengths without sagging). Sold in 3m lengths as standard, though 6m lengths are available from trade merchants like Wolseley if you need long unjointed runs.

The "28mm" measurement is the outside diameter, not the bore. This is metric sizing. It matters because compression fittings, solder ring fittings, and clips are all sized by OD.

In a typical extension, 28mm copper appears in a handful of specific places:

• Boiler flow and return on systems above approximately 23 kW output, where 22mm would restrict water flow
• Primary circuit to a hot water cylinder on larger systems
• Air source heat pump primary circuits, where lower operating temperatures mean higher flow volumes and 22mm creates too much resistance
• Gas supply runs (must be installed by a Gas Safe registered engineer, not a general plumber)
• Incoming mains water supply in very high-demand properties, though the incoming service pipe from the street is usually 25mm or 32mm MDPE, not copper

Most domestic plumbing uses 15mm and 22mm copper. 28mm is the exception, not the rule. A standard three-bedroom house with a combi boiler and 8-10 radiators runs entirely on 22mm and 15mm. You only step up to 28mm when the system demands it.

When do you actually need 28mm?

This is the question that generates the most confusion on plumbing forums. The answer comes down to one number: heat carrying capacity.

28mm copper pipe at a standard domestic flow velocity of 1.5 metres per second with a 20-degree temperature drop between flow and return can carry 67.5 kW of heat. That's enough for any domestic boiler currently on the UK market. By comparison, 22mm pipe carries around 30 kW under the same conditions.

The practical rules:

You need 28mm from the boiler tappings to at least the first tee or diverter valve on any boiler rated above approximately 23 kW. Running 22mm right from the boiler on a 30 kW system creates a bottleneck that causes circulation problems and can trigger overheat lockouts.

You need 28mm on air source heat pump primary circuits. Heat pumps operate at lower flow temperatures than gas boilers (typically 35-45 degrees C versus 60-75 degrees C), which means they need to push more water around the system to deliver the same heat output. The pressure drop through 22mm pipe at ASHP flow rates is 426 Pa/m. Through 28mm, it drops to 124 Pa/m. That's a big reduction in pump energy and noise.

You don't need 28mm for radiator feeds. Individual radiators are fed in 15mm (most) or 22mm (very large radiators). Nobody feeds a single radiator in 28mm.

You don't need 28mm for a small combi boiler system with fewer than 12-14 radiators. 22mm handles this comfortably.

Your plumber should be sizing the pipe to match the system, not defaulting to 28mm everywhere "just in case." If someone quotes you 28mm throughout the house and you have a standard combi system, ask why.

Types and specifications

All 28mm copper pipe sold in the UK is manufactured to the same standard. There's no meaningful variation between brands for domestic work.

The weight matters more than you'd think. A 3m length of 28mm copper weighs about 2 kg. Not heavy enough to be a problem carrying it, but noticeably stiffer and harder to manoeuvre in tight spaces than 15mm or 22mm. Bending 28mm by hand is not possible. You need a mechanical pipe bender with a 28mm former, or you use fittings at every change of direction.

Pipe clips must be spaced at maximum 1.5m intervals on horizontal runs and 2.4m on vertical runs.

How to work with it

Cutting

Cut 28mm copper with a pipe cutter (also called a pipe slice), not a hacksaw. A hacksaw leaves a rough edge that creates a burr inside the bore, which restricts flow and prevents fittings from sealing properly. A rotary pipe cutter for 28mm costs around £8 – £15 and gives a clean, square cut every time. After cutting, always ream the inside edge with the triangular reamer blade built into most pipe cutters to remove the slight internal lip.

Joining methods

28mm is where joining copper gets more involved than the smaller sizes. Five methods exist, but only two are common in domestic work.

Compression fittings are the standard choice for 28mm in accessible locations. The fitting uses an olive (a soft brass or copper ring) compressed between the nut and the body to create a watertight seal. No heat, no flux, no specialist tools beyond two adjustable spanners. The two-spanner technique is mandatory at this size: hold the fitting body with one spanner while tightening the nut with the other. Recommended torque is 80 Nm, or hand-tight plus half to three-quarter turn. Do not use PTFE tape on the olive.

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Solder ring (capillary) fittings are the professional choice for permanent, hidden runs. The fitting has a ring of solder pre-loaded inside it. You clean the pipe end and fitting socket with wire wool, apply flux, push together, and heat with a propane or butane torch until the solder ring melts and wicks around the joint by capillary action. You'll see a bright ring of solder appear at the edge of the fitting when it's done. Solder ring fittings are cheaper per unit than compression (roughly £2 – £3 each at 28mm) and produce a lower-profile joint that's easier to conceal. But they require more skill, a gas torch, and careful preparation. Any contamination (grease, dirt, oxidation) on the mating surfaces will cause a failed joint.

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End-feed capillary fittings are even cheaper but require you to add solder wire separately. More skill again. Not recommended unless you've soldered copper before.

Press-fit fittings (brands like Viega Propress, Mapress) use a special crimping tool to deform a stainless steel sleeve around the pipe. Fast and reliable, but the tool costs hundreds of pounds. Your plumber may use this method. You won't be buying the tool for a single project.

Push-fit fittings technically exist at 28mm. Hep2O and Speedfit both manufacture them. But 28mm is the absolute maximum size available in push-fit, and professional plumbers rarely use them at this diameter. The applications that call for 28mm (boiler primaries, ASHP circuits) involve higher pressures and temperatures where compression or solder ring is the professional convention. Push-fit at 28mm is not wrong, but it's unusual.

Bending

Forget about bending 28mm by hand. Unlike 15mm pipe, which a competent person can bend with a handheld bending spring, 28mm copper is too stiff and too thick-walled. You need a mechanical pipe bender with a 28mm former (manufacturers include Ridgid and Rothenberger). These cost £80 – £150 to buy or £15 – £25 per day to hire.

The practical recommendation for a homeowner overseeing an extension: don't worry about bending. Your plumber will either use a bender or use elbow fittings at every change of direction. Elbow fittings add cost (£4 – £5 each) but are faster than setting up a bender for a handful of bends.

Sleeving through masonry

Copper in direct contact with concrete, cement mortar, or limestone masonry corrodes. Within 2-3 years, the pipe wall thins and eventually leaks. Every pipe passing through a wall or floor must be sleeved in a plastic duct or wrapped in suitable tape. This is an NHBC requirement and building control will check for it at first fix inspection.

The first metre from the boiler

The first 800-1000mm of pipework from any boiler must be copper, not plastic. During a boiler overshoot event (a brief spike in temperature and pressure when the boiler fires and the pump hasn't caught up), temperatures at the boiler tappings can momentarily exceed the safe working temperature of plastic pipe and push-fit fittings. Copper handles this without issue. This applies regardless of what the rest of the system is plumbed in.

The imperial/metric trap

If you're extending the heating system in a house built before the early 1970s, there's a specific hazard to know about. Britain switched from imperial to metric pipe sizes during that period. Old "1 inch" copper pipe has an outside diameter of approximately 31.8mm. Modern "28mm" copper pipe has an outside diameter of 28mm. They are not the same.

People commonly refer to 28mm pipe as "one inch copper." Colloquially, this is fine. But if you're physically connecting new 28mm pipe to existing 1 inch imperial pipe in an older property, the two do not fit together without a specific imperial-to-metric adaptor fitting (28mm metric to 1 inch imperial compression). These are available from plumbing merchants but not always stocked at Screwfix or Toolstation. Know this before your plumber arrives on site, because a missing adaptor can hold up a morning's work.

Cost and where to buy

28mm copper pipe is expensive relative to the smaller sizes. Copper is a commodity metal and the price tracks global markets, which have been near record highs through 2024-2025.

Current UK retail pricing for a 3m length of 28mm copper pipe (March 2026, inc. VAT):

That works out to roughly £10 – £13 per metre at retail. Cut lengths from specialist suppliers run around £14 – £15 per metre (you pay a premium for the convenience of buying exactly what you need rather than a full 3m length).

For context, 15mm copper pipe costs around £13 – £14 per 3m, and 22mm costs around £26 – £30 per 3m. The jump to 28mm is still significant, which is exactly why you want to make sure it's only used where the system genuinely needs it.

Fittings add up fast at 28mm. Budget £3 – £5 per straight coupler, £4 – £5 per elbow, and £5 – £9 per tee. A typical boiler connection with four elbows, two tees, and a pair of full-bore isolating valves can easily run £40 – £60 in fittings alone, before labour.

How much do you need

For a typical extension with a relocated or new boiler and a wet central heating system, 28mm usage is limited to short runs:

• Boiler flow and return to first tee or diverter valve: 1-3 metres each side, so 2-6 metres total
• ASHP primary circuit (if applicable): 3-10 metres depending on where the unit sits relative to the cylinder or buffer
• Gas supply run (if applicable, Gas Safe only): varies by distance from meter to boiler

A realistic material order for a single-storey extension with a boiler relocation is 2-3 lengths of 3m pipe (6-9 metres total), plus 8-12 compression or solder ring fittings. Total material cost for the 28mm portion: £80 – £160 depending on fittings count and retailer.

Wastage on copper pipe is minimal because offcuts from one run typically get used elsewhere. Allow 5% wastage at most. Don't over-order; 28mm is expensive enough that buying an extra 3m length "just in case" means £30+ sitting unused.

Alternatives

22mm copper pipe handles the majority of domestic heating and hot water work. If your system is a standard combi boiler under 28 kW serving a modest number of radiators, 22mm is probably all you need. The cost saving is considerable: roughly half the price per metre for pipe, and fittings are cheaper too.

Plastic barrier pipe (Hep2O, Speedfit, or similar polybutylene systems) is a viable alternative for long hidden runs where you want to minimise joints. Available in 22mm and 28mm, though 28mm plastic has a smaller internal diameter than 28mm copper, which matters for pressure-sensitive ASHP circuits. If your heating designer specified 28mm copper for an ASHP primary, switching to 28mm plastic may require upsizing to 32mm barrier pipe to maintain adequate flow. Don't make this substitution without consulting the designer.

Plastic is not a substitute for the first metre from the boiler. Regardless of what the rest of the system uses, the immediate boiler connections should be copper.

Where you'll need this

• First fix plumbing - boiler flow/return connections, primary heating circuit runs, gas supply pipework

28mm copper appears during the first fix phase of any project that involves boiler installation, relocation, or heating system modification. It's a specialist material for specific high-flow applications, not a general-purpose pipe size. Your plumber will know where it's needed. Your job is knowing enough to check that the specification makes sense for your system.

Common mistakes

Running 28mm where 22mm is sufficient. Over-specifying pipe size doesn't improve system performance. It wastes money on pipe, fittings, and the extra labour time to work with a larger, stiffer material. If your plumber specifies 28mm throughout and you have a standard domestic system, ask them to explain why each section needs the larger bore.

Using push-fit fittings on boiler primaries. Push-fit fittings at 28mm technically work, but the applications that call for 28mm (high flow, higher pressure, close to heat sources) are exactly where compression or solder ring is the safer choice. Most professional plumbers wouldn't use push-fit here.

Failing to sleeve through masonry. Copper corrodes on contact with cement and concrete. Every penetration through a wall or floor needs a plastic sleeve. Building control will flag this if they spot it, and rightly so, because unprotected copper in concrete can fail within 2-3 years.

Connecting 28mm to old 1 inch imperial pipe without an adaptor. The two are different sizes. Forcing a 28mm compression fitting onto 1 inch imperial pipe will not seal. You need a specific metric-to-imperial adaptor. Check before the plumber arrives.

Burying compression joints in floors or walls. Compression fittings are mechanical joints that can be adjusted or replaced. Embedding them in screed or concrete removes that option. If a compression joint in a floor ever weeps, you're breaking out concrete to reach it. Use solder ring for any joint that will be concealed permanently.